Gas springs are widely used in automotive vehicles to lift and partly or wholly counterbalance engine compartment hoods, trunk lids, tail gates and the like (hereinafter referred to as "loads"). To prevent the loads from moving toward open at an undesirably rapid speed, most gas springs used in automotive and similar applications incorporate a dampening feature in the form of a piston unit having a restrictive bypass that throttles the flow of fluid across the piston unit when the piston rod is moving out to lift the load. At the same time it is usually desirable to have an open bypass across the piston unit when the load is being moved toward closed in order to keep the handle load low. Gas springs having a restricted bypass that operates when the piston rod moves out and an open bypass that operates when the piston rod moves in are described and shown in U.S. Pat. Nos. 4,438,833 (Schafer, Mar. 27, 1984), 4,466,514 (Molders et al., Aug. 21, 1984), and 4,467,899 (Molders et al., Aug. 28, 1984).
The gas springs of those patents comprise an elongated tube that is closed at one end and receives a piston rod in sealed and sliding relation at the other end and a piston unit affixed to the piston rod within the tube and defining a closed end chamber within the tube adjacent said one end and a rod end chamber within the tube adjacent said other end. The tube contains a gas at a pressure greater than atmospheric pressure and usually contains some liquid, such as brake fluid. A restricted bypass through the piston unit throttles the flow of the gas and liquid between the closed end and rod end chambers, and a one-way open bypass defined by an annular clearance space between a perimeter wall of the piston unit and the tube adjacent the closed end chamber and at least one passage in the piston unit adjacent the rod end chamber allows essentially unrestricted fluid flow from the closed end chamber to the rod end chamber A sealing ring received in a ring groove in the piston unit intermediate the clearance space and passage and engaged in sealed and sliding relation with the inner wall of the tube is movable in the ring groove axially with respect to the tube between a sealed position in engagement with a ringland adjacent the clearance space such as to close the open bypass and an open position spaced apart from the ringland such as to open the bypass.
To enable the fluid in the closed end chamber to flow through the open bypass when the sealing ring is in the open position, the inner edge of the sealing ring is relatively widely spaced apart from the base of the ring groove to provide an annular flow passage that forms part of the open bypass--i.e., communicates the clearance space between the piston unit and the inner wall of the tube adjacent the closed end chamber with the passages in the piston unit adjacent the rod end chamber.
To ensure a good sealing engagement between the sealing ring and the ringland in the closed position of the sealing ring so that fluid does not leak through the open bypass when the rod is moving out and instead must flow through the restricted bypass, it is necessary for the outer circumference of the ringland to be in close clearance from the inner wall of the tube and for the radial dimension of the zone of engagement between the ringland and the sealing ring to exceed a certain value at all points circumferentially.
In many applications of gas springs the tube diameter must be kept as small as possible to conserve space. Keeping the diameter small presents various difficulties, such as providing an adequate land area width for the sealing ring and sufficient clearance between the piston and the tube to keep the piston from rattling against the tube wall